More particularly, the invention relates to an annular vane assembly for a gas turbine engine, the assembly including a vane segment comprising an arcuate rail and at least one vane that extends radially inwardly from the arcuate rail, the assembly also including a hollow cylindrical casing in the inside curved surface of which is formed an annular groove for receiving the arcuate rail of the vane segment.
One known vane segment 1 is shown in FIG. 1a, and comprises a radially inner arcuate rail 3, a radially outer arcuate rail 5, and vanes 7 that extend radially between the inner and outer rails. The outer rail 5 has flanges 9 that run along either side of the rail. One known hollow cylindrical casing 11 is shown in FIG. 1b, and includes in its inside curved surface 13 a plurality of annular grooves 15. Each annular groove 15 has recesses 17 that run along either side of the groove.
The vane segment 1 of FIG. 1a is fitted to the casing 11 of FIG. 1b by aligning the ends of the flanges 9 of the outer rail 5 of the vane segment with the ends of the recesses 17 of an annular groove 15 of the casing, and sliding the flanges circumferentially around the recesses so that the outer rail slides circumferentially around the annular groove. FIG. 1c shows the mating relationship between the outer rail 5 and the annular groove 15 when the vane segment 1 is fitted to the casing 11.
The known annular vane assembly of FIGS. 1a to 1c is an assembly of a compressor of a gas turbine engine.
There are various mechanisms by which vane segment 1, once fitted to casing 11, can be secured in place.
One such mechanism is as shown in FIG. 1c. The flanges 9 are a tight fit within the recesses 17, i.e. there is a minimum clearance between the radially inwardly/outwardly facing surfaces of the flanges/recesses, thereby to hold the vane segment 1 at a predetermined position in the radial direction. This mechanism, although low cost, gives rise to problems in assembly if there has been minor distortion in the physical form of the vane segment during its fabrication. Also, if it is required to remove the vane segment from the casing following actual in service use of the gas turbine engine, then this can be very difficult due to corrosion and distortion of the vane segment during use.
Another mechanism is as shown in FIG. 2. The annular grooves 15 are formed by clamp rings 19 bolted to the inside curved surface 13 of the hollow cylindrical casing 11 by means of bolts (not shown) that pass via holes 21 from the outside of the casing to the clamp rings. Removal of vane segments is made easy by removal of the clamp rings. This mechanism, although solving the problems of the FIG. 1c mechanism, is expensive.
A further mechanism is shown in FIG. 3. The cross section of the annular groove 15 is such as to loosely fit the radially outer arcuate rail 5 of the vane segment 1, and a spring pack 23 is used to secure the flanges 9 of the rail 5 against the radially outwardly facing surfaces 25 of the recesses 17 of the groove 15. The spring pack 23 comprises a spring 27, a spring holder 29, and a jacking screw 31. Tightening of jacking screw 31 causes spring holder 29 to bear down upon flanges 9, clamping flanges 9 onto surfaces 25 with a controlled spring load. Vane segment 1 is now secured in position. In use temperature change may give rise to relative movement between constituent parts. The controlled spring load allows some such movement. Loosening of jacking screw 31 unclamps flanges 9, releasing vane segment 1 for removal from annular groove 15. Typically two or three spring packs 23 are used per vane segment. The mechanism of FIG. 3 suffers from the disadvantage that it is complex.